A pressure gauge on the pump has failed; the pointer is crooked and wobbling. There is a replacement in the cupboard with a matching thread. After fitting, the pointer shakes so much that nobody can read the pressure, and after a few weeks this gauge is broken too. The thread was right; the rest was not.
A pressure gauge is a simple measuring instrument, but not a one-size-fits-all part. Measuring range, accuracy, materials, filling and connection must suit the measuring point. If you go by the thread alone, you measure inaccurately or replace gauges more often than necessary.
This checklist shows the seven details that must be right when buying a pressure gauge – from the measuring range to the safety design. It helps with new purchases as well as with replacing an existing gauge.
Briefly explained: how a pressure gauge measures
Most mechanical pressure gauges work with an elastic measuring element that deforms under pressure. A movement transfers this deformation to the pointer. Depending on the measuring range and application, three measuring elements are used:
- Bourdon tube: a curved tube that straightens out under pressure. The standard for medium and high pressures.
- Diaphragm element: a corrugated diaphragm between two flanges. Good for low pressures, viscous or contaminated media and for high overload safety.
- Capsule element: two diaphragms welded together. For very low pressures in the millibar range, especially with gases.
Mechanical Bourdon tube pressure gauges are covered by EN 837-1, diaphragm pressure gauges by EN 837-3. EN 837-2 gives recommendations on selection and installation.
Detail 1: Type of pressure and measuring range
First the question of which type of pressure is being measured:
- Gauge pressure: relative to atmospheric pressure. The normal case in hydraulics, pneumatics and water.
- Negative pressure: a vacuum gauge shows, for example, from −1 to 0 bar. For suction lines there are compound gauges that cover both ranges.
- Absolute pressure: relative to a perfect vacuum. Important when fluctuations in atmospheric pressure must not distort the result.
- Differential pressure: the difference between two pressures, for example upstream and downstream of a filter.
Then the measuring range. A proven rule: with steady pressure, the normal operating pressure should be no more than about three quarters of the full-scale value; with fluctuating pressure, no more than about two thirds. That leaves a reserve for pressure peaks, and the Bourdon tube is not permanently loaded at its limit. Too large a measuring range is also unfavourable, because the pointer then only uses a small part of the scale and the reading becomes inaccurate.
Fictitious example calculation: a hydraulic system works at around 100 bar with brief pressure peaks. A pressure gauge with 0 to 160 bar is a good fit. A gauge with 0 to 100 bar would be permanently at its stop, one with 0 to 600 bar too coarse.
Detail 2: Nominal size
The nominal size is the case diameter in millimetres. Common sizes are 40, 50, 63, 80, 100 and 160 mm. Small sizes are fitted to pressure reducers, air preparation units and compact equipment. Larger gauges are easier to read from a distance and are usually available in higher accuracy classes. When replacing: the nominal size also determines whether the gauge fits the existing bracket or panel cut-out.
Detail 3: Accuracy class
The accuracy class indicates the permissible measuring error as a percentage of the full-scale value. EN 837-1 specifies, among others, the classes 0.1, 0.25, 0.6, 1.0, 1.6, 2.5 and 4.0.
Fictitious example calculation: a 0 to 16 bar pressure gauge of class 1.6 may deviate by up to 0.256 bar – across the entire range, i.e. also at low pressures. This shows once again why a suitable measuring range is important.
| Class | Typical use |
|---|---|
| 0.1 to 0.6 | Test and reference gauges, calibration |
| 1,0 | Operational measurements with higher requirements |
| 1,6 | Standard in machine and plant engineering |
| 2.5 and 4.0 | simple indication, air preparation units, small nominal sizes |
Detail 4: Connection and design
This is where most mix-ups happen. Check:
- Thread: parallel threads such as G 1/4 or G 1/2 are common in Europe, tapered NPT threads mainly on equipment from North America. Both look similar but seal differently and are not readily interchangeable.
- Connection position: bottom entry, i.e. pointing vertically downwards, or back entry, i.e. axially to the rear. Back connections are often centred, sometimes off-centre.
- Mounting: directly on the line, with a rear flange for wall mounting or with a front flange for panel mounting.
Detail 5: Materials and medium
The measuring element is in direct contact with the medium. Standard gauges have copper alloy wetted parts and are suitable for air, water and oils. For aggressive media, ammonia, seawater or food, you need wetted parts and a case made of stainless steel (CrNi steel).
For very aggressive, viscous, crystallising or hygienically sensitive media, a diaphragm seal is used. A diaphragm separates the instrument from the medium, and a fill fluid transmits the pressure.
Temperature matters too. Hot media such as steam must not reach the measuring element directly. A siphon upstream of the pressure gauge ensures that condensate forms as a protective cushion.
Detail 6: Liquid filling and damping
Strong vibrations and pulsating pressure, for example on piston pumps, compressors or hydraulic power units, make the pointer shake and wear out the movement. A case filling, usually glycerine or silicone oil, damps the pointer and protects the movement. A restrictor screw in the connection or a pulsation damper can also help.
That is exactly the mistake from the introduction: at a pulsating measuring point, a liquid-filled gauge with a suitable measuring range would have lasted a long time. Note that the fill fluid has a temperature range and becomes more viscous in the cold.
Detail 7: Safety and additional functions
- Safety design: with high pressures and hazardous media, a design with a solid baffle wall and blow-out back protects the operator if the measuring element fails. EN 837-1 describes the safety pattern S3 for this.
- IP rating: for damp or dusty environments, specify the degree of protection according to IEC 60529, for example IP65.
- Drag pointer: shows the highest pressure reached.
- Limit contacts or electrical output signal: if the pressure gauge is also to switch or report to a control system.
- Calibration certificate: if the gauge is managed as test equipment in your measuring equipment monitoring.
Checklist to copy
| Detail | Your entry |
|---|---|
| Type of pressure and measuring range | gauge / negative / absolute / differential, from … to … bar |
| Nominal size | … mm |
| Accuracy class | … |
| Connection | thread, bottom or back entry, mounting |
| Materials and medium | wetted parts, case, medium, temperature |
| Filling and damping | dry / filled, restrictor yes / no |
| Safety and extras | safety design, IP rating, contacts, calibration certificate |
Five typical mistakes
- Measuring range too tight: the operating pressure is at the end of the scale, and pressure peaks permanently bend the Bourdon tube. The gauge then reads too high or no longer returns to zero.
- Dry gauge at a pulsating measuring point: the pointer shakes and the movement wears out in a short time.
- Thread mixed up: an NPT gauge in a G thread does not seal properly, and if it is forced in, the connection is damaged.
- Material not resistant: copper alloy wetted parts on an aggressive medium corrode from the inside without it being visible from the outside.
- Unit overlooked: a scale in psi instead of bar, or in kPa instead of mbar, leads to reading errors, especially when several gauges hang side by side on site.
Replacement: the details from the old gauge
When replacing, you will find a lot of data directly on the dial: manufacturer, measuring range, unit, accuracy class and often a symbol for the measuring element. The type and item number are frequently on the case or the back. Photograph the dial, back and connection, and measure the thread if there is no marking.
If the pressure gauge is managed as test equipment, the new gauge usually needs its own certificate. How calibration intervals and records are organised is explained in our guide to calibrating measuring equipment.
When a specific pressure gauge is needed
Machines from abroad in particular contain pressure gauges with NPT threads, scales in psi or special materials that are not in every catalogue. KTB Europe is a family business from Hamburg and has been sourcing industrial spare parts of all brands worldwide since 1976. Send us the part number, a photo of the dial and the nameplate, or a description with the seven details. If a type has been discontinued, we look for equivalent alternatives. Before dispatch, we inspect the parts in our own central warehouse.
Find out more on the pages on MRO procurement and order consolidation. You can send your enquiry via the contact page. If you measure negative pressure, you will find further advice in our guide to the industrial vacuum pump.
Conclusion
You cannot recognise a suitable pressure gauge by its thread alone. Type of pressure and measuring range, nominal size, accuracy class, connection, materials, filling and safety design must all suit the measuring point. If you clarify these seven details before buying, you will measure more accurately, read more easily and replace less often.
Frequently asked questions
What measuring range should a pressure gauge have?
With steady pressure, the normal operating pressure should be no more than about three quarters of the full-scale value; with fluctuating pressure, no more than about two thirds. That leaves a reserve for pressure peaks. A measuring range that is far too large, on the other hand, makes the reading inaccurate.
What does the accuracy class of a pressure gauge mean?
The class indicates the permissible deviation as a percentage of the full-scale value. A gauge with 0 to 10 bar and class 1.0 may therefore deviate by up to 0.1 bar, across the entire measuring range. For most operational measurements, class 1.0 or 1.6 is usual.
When do I need a liquid-filled pressure gauge?
Whenever vibrations or pulsating pressure occur, for example on piston pumps, compressors or hydraulic power units. Filling with glycerine or silicone oil damps the pointer and protects the movement from wear. This keeps the value readable and makes the gauge last longer.
What is the difference between G and NPT threads on a pressure gauge?
G threads are parallel and seal via a gasket; NPT threads are tapered and seal in the thread itself. G threads are widespread in Europe, NPT mainly on equipment from North America. The two are not readily interchangeable, even if they look similar.
What details can I find on the old pressure gauge for the replacement?
The dial usually shows the manufacturer, measuring range, unit and accuracy class, often also a symbol for the measuring element. The type and item number are frequently on the case or the back. Add photos and the measured connection thread.